Wireless LANs are receiving attention as systems which release users from the LAN wiring required in wired methods. According to wireless LANs, since almost all wired cables can be omitted in work spaces such as offices, communication terminals such as personal computers (PCs) can be comparatively easily moved. In recent years, as wireless LAN systems have increased in speed and decreased in price, demand for wireless LAN systems have remarkably increased. In recent years, consideration is particularly given to installation of personal area networks (PANs) in order to construct a small-scale wireless network and perform information communication among a plurality of personal electronic devices.
For example, a method called ultra-wideband (UWB) communication, which performs wireless communication by using an extremely wide frequency bandwidth, is recently receiving attention as a wireless communication system which realizes short-distance ultra-high-speed transmission, and such method is expected to be put to practical use. The IEEE802.15.3 committee and the like have presently proposed a data transfer method based on a packet structure containing preambles, as an access control method for ultra-wideband communication.
In UWB communication, modulation methods such as DS-SS and OFDM are considered. According the DS-SS method, during transmission, an information signal is multiplied by a random code sequence called PN (Pseudo Noise) code to directly spread (DS: Direct Spread) the occupied bandwidth, and on a reception side, the received spread information signal is reproduced into the information signal by being inversely spread by being multiplied by the PN code. The DS-SS method makes it possible to realize high-speed data transfer by performing transmission and reception in a spread ultra-high frequency bandwidth of, for example, 3 GHz to 10 GHz.
According to the OFDM (Orthogonal Frequency Division Multiplexing) method, the frequency of each carrier is set so that each carrier is orthogonal with every other carrier in a symbol duration, and during transfer of information, plural pieces of data are allocated to the respective carriers and modulation is performed on the amplitude and phase of each of the carriers, and inverse FFT is performed on the plural carriers to convert each of the carriers into a signal on the time axis with orthogonality on the frequency axis being maintained, and the signal on the time axis is transmitted. During reception, FFT is performed to convert the signal on the time axis into a signal on the frequency axis and the respective carriers are subjected to demodulation corresponding to their modulation methods to reproduce the information transmitted by the original serial signal. Since transmit data are transmitted in the state of being distributed to a plurality of carriers of different frequencies, the bandwidth of each of the carriers is narrowed narrow and becomes resistible to frequency selective fading.
In wireless communication equipment, during reception of wireless signals, it is general practice to perform voltage amplification on received signals. For example, in the above-mentioned ultra-wideband communication, voltage amplification is performed on a high frequency component by a low noise amplifier (LNA). In this case, it is desired that voltage amplification is collectively performed in a wide frequency bandwidth extending over a 2-GHz range of 3 GHz to 5 GHz which is used in UWB.
Wide-band amplifiers can be generally constructed by a combination of an amplifier device made of MOS-FETs (Metal Oxide Semiconductor-Field Effect Transistors), bipolar transistors or the like, and a band-pass filter (refer to, for example, Non-Patent Document 1).
FIG. 8 shows the construction of a wide-band amplifier constructed by a combination of an amplifier device and a first-order band-pass filter (BPF) (refer to, for example, Non-Patent Document 2).
As shown, the wide-band amplifier is constructed so that a first-order band-pass filter made of an LC parallel resonant circuit made of a parallel coil Lp 103, a parallel capacitor Cp 104 and a resistor RL 105 is provided as a load in parallel with the drain and the source of an amplifier device 102 constructed with MOS-FETs or bipolar transistors.
In FIG. 8, reference numeral 101 denotes an input terminal of the wide-band amplifier, and reference numeral 108 denotes an output terminal of the wide-band amplifier, and the amplifier device 102 operates as a voltage-controlled current source. Specifically, a voltage V1 at the input terminal 101 is applied to the gate of the amplifier device 102, and the amplifier device outputs a current of gm times the gate voltage V1 in the direction indicated by an arrow in FIG. 8. The voltage provided at the output terminal 108 at this time is denoted by V2.
The transfer function H(s) of the wide-band amplifier shown in FIG. 8 is expressed by the following formula:
                              H          ⁡                      (            s            )                          =                                            -              s                        ·            Lp            ·            RL            ·            gm                                                              s                2                            ·              Lp              ·              Cp              ·              RL                        +                          s              ·              Lp                        +            RL                                              [                  Formula          ⁢                                          ⁢          1                ]            
FIG. 9 shows a pole-zero map in the s-plane of the wide-band amplifier shown in FIG. 8. In FIG. 9, the symbol “o” denotes a zero, and the symbol “x” denotes a pole. On the s-plane, poles are located at points where the denominator of the transfer function H(s) is 0, while zeroes are located at points where the numerator of the transfer function H(s) is 0. In the shown example, a zero is located at the center of the s-plane, and the number of poles that corresponds to the order of the band-pass filter appear on one side of the s-plane.
FIGS. 10 and 11 respectively show a transfer characteristic example and a group delay characteristic of the wide-band amplifier shown in FIG. 8. Each of the characteristics is normalized as gm×RL=1 with a center frequency of 4 GHz. A cross section obtained on the imaginary axis when the band-pass filter has a transfer characteristic of −∞ at the zero and a transfer characteristic of +∞ at each of the poles on the s-plane corresponds to the transfer characteristic of the band-pass filter. The band-pass filter in which the parameter of the LCR parallel resonant circuit (i.e., the s-plane transfer characteristic shown in FIG. 9) is set to flatten the passband (for example, 3 GHz to 5 GHz) is called a Butterworth filter, and such transfer characteristic is called a Butterworth character.
However, as can also be seen from FIGS. 10 and 11, in the wide-band amplifier constructed using the first-order band-pass filter made of the LCR parallel resonant circuit as a load for the amplifier device, the following problems arise.
(1) The frequency characteristic is a single peak characteristic and does not have flatness sufficient to be used in a wide bandwidth. This problem also depends on the fact that the first-order band-pass filter merely has the number of poles that corresponds to the order, i.e., one pole, on one side.
(2) The amplifier has a comparatively simple construction as shown in FIG. 8; nevertheless the wide-band amplifier has group delay time.
In this construction, if the bandwidth over which the flatness is to be maintained is to be widened, the inductance Lp of the coil 103 must be increased, or the resistance value RL of the resistor 105 must be decreased. However, if the inductance Lp is increased, since the self-resonant frequency is low, the amplifier is not suitable for operation at high frequencies. In addition, if the resistance value RL is decreased, the amplifier decreases in gain.
FIG. 12 shows the construction of a wide-band amplifier constructed by a combination of an amplifier device and a second-order band-pass filter (BPF).
As shown, the wide-band amplifier is constructed so that a second-order band-pass filter is provided as a load in parallel with the drain and the source of the amplifier device 102 constructed with MOS-FETs or bipolar transistors. The second-order band-pass filter is constructed with an LC parallel resonant circuit made of the parallel coil Lp 103 and the parallel capacitor Cp 104, an LC series resonant circuit made of a series coil Ls 107 and a series capacitor Cs 106, and the resistor RL 105.
FIG. 12, reference numeral 101 denotes an input terminal of the wide-band amplifier, and reference numeral 108 denotes an output terminal of the wide-band amplifier, and the amplifier device 102 operates as a voltage-controlled current source. Specifically, a voltage V1 at the input terminal 101 is applied to the gate of the amplifier device 102, and the amplifier device outputs a current of gm times the gate voltage V1 in the direction indicated by an arrow in FIG. 12. The voltage provided at the output terminal 108 at this time is denoted by V2.
The transfer function H(s) of the wide-band amplifier shown in FIG. 12 is expressed by the following formula:
                              H          ⁡                      (            s            )                          =                                            -                              s                2                                      ·            Lp            ·            Cs            ·            RL            ·            gm                                                                                                                          s                      4                                        ·                    Lp                    ·                    Ls                    ·                    Cp                    ·                    Cs                                    +                                                            s                      3                                        ·                    Lp                    ·                    Cp                    ·                    Cs                    ·                    RL                                    +                                                                                                                                                s                      2                                        ·                                          (                                                                        Lp                          ·                          Cp                                                +                                                  Ls                          ·                          Cs                                                +                                                  Lp                          ·                          Cs                                                                    )                                                        +                                      s                    ·                    Cs                    ·                    RL                                    +                  1                                                                                        [                  Formula          ⁢                                          ⁢          2                ]            
FIG. 13 shows a pole-zero map in the s-plane of the wide-band amplifier shown in FIG. 12. In FIG. 13, the symbol “o” denotes a zero, and the symbol “x” denotes a pole. On the s-plane, poles are located at points where the denominator of the transfer function H(s) is 0, while a zero is located at a point where the numerator of the transfer function H(s) is 0. In the shown example, a zero is located at the center of the s-plane, and two poles corresponding to the order of the band-pass filter appear on one side of the s-plane. In this wide-band amplifier, the Butterworth character is adopted in order to flatten the passband (for example, 3 GHz to 5 GHz).
FIGS. 14 and 15 respectively show a transfer characteristic example and a group delay characteristic of the wide-band amplifier shown in FIG. 12. Each of the characteristics is normalized as gm×RL=1 with a center frequency of 4 GHz.
In the wide-band amplifier constructed using the second-order band-pass filter as a load for the amplifier device, as can also be seen from a comparison of FIGS. 10 and 14, the characteristic flatness in the passband is improved, but the following problems arise.
(1) The wide-band amplifier shown in FIG. 12 is long in group delay time compared to FIG. 11, and is not suitable for a voltage-feedback amplifier circuit. This problem depends on the fact that the series coil Ls 107 and the series capacitor Cs 106 are inserted in series between the output terminal of the amplifier device 102 and the output terminal 108 of the amplifier and the resonance of this LC circuit becomes a main cause of delay. (In the case of the amplifier shown in FIG. 8, since the output terminal of the amplifier device 102 serves as the output terminal 108 of the amplifier, the problem of group delay does not arise.)
(2) In the case where a subsequent-stage circuit (such as a down-converter, an AGC or an A/D converter) is connected to the output terminal 108 of the amplifier, the circuit acts as parasitic capacitance for the amplifier, but since a capacitance element does not exist between the output terminal of the amplifier and GND, the parasitic capacitance added to the output terminal cannot be absorbed as part of constants and the frequency characteristic degrades.
In the example shown in FIG. 12, since the amplifier is constructed so that the series capacitor Cs 106 and the parasitic capacitance are connected in series, it is difficult to eliminate the influence of parasitic capacitance. On the other hand, in the example shown in FIG. 8, the parallel capacitor Cp and the parasitic capacitance are connected in parallel instead of the construction in which the capacitor is connected in series with the parasitic capacitance, so that it is possible to easily eliminate the problem of parasitic capacitance by decreasing the capacitance of the parallel capacitor Cp 104.    Non-Patent Document 1    Ken Yanagisawa and Noriyoshi Kamiya, “Theory and Design of Filter”, (Akiba Shuppan, 1986)    Non-Patent Document 2    Guillermo Gonzales, “Microwave Transistor Amplifiers Analysis and Design”, (pp. 170-172, Prentice Hall, 1984)